A three-way catalyst positioned in an engine exhaust path provides both oxidation and reduction characteristics to lower tailpipe emissions for hydrocarbons, carbon monoxide, and nitrogen oxides (NOx). When the engine is stopped, the exhaust flow through the catalyst is halted and the catalyst acts like a sponge, thus accumulating oxygen. The longer the engine is off, the more oxygen is accumulated by the catalyst, up to the catalyst's oxygen saturation point, which is a function of catalyst temperature. This additional oxygen stored during the engine stop may be compensated for during the ensuing engine start. Without compensation, the catalyst's ability to reduce NOx will be significantly impacted. In order to compensate for the stored oxygen, additional fuel may be added on the restart to “reactivate” the catalyst's reduction capability. This assures that the generated emissions are minimized during every engine restart.
Many engine off events result in the catalyst becoming saturated with oxygen. As such, most engine restarts are performed under the assumption that the catalyst is saturated, and thus a relatively high amount of fuel is provided to reactivate the catalyst. However, during certain engine restarts, such as following an automatic engine stop, the engine off duration may be relatively short, resulting in only partial catalyst saturation. Performing a standard reactivation with the relatively high fuel enrichment on a partially saturated catalyst may result in more fuel than necessary being supplied to the catalyst, increasing hydrocarbon emissions and wasting fuel.
The inventors have recognized the issues with the above approach and offer a method to at least partly address them. In one embodiment, a method for reactivating a catalyst coupled to an engine comprises when the engine is restarted following deactivation, adjusting a degree of fuel enrichment based on an engine off duration and an engine air amount during the restart.
In this way, the amount of oxygen stored in the catalyst may be approximated based on the duration of the engine off period following engine deactivation. The degree of enrichment provided to the engine during the restart may be based on the engine off period to convert the stored oxygen and thus reactivate the catalyst. Further, one or more parameters of the enrichment, such as the relative richness and/or duration of the enrichment, may be adjusted based on the air flow to the engine. Accordingly, the enrichment may be matched to the air flow to provide a precise amount of extra fuel to the engine to reactivate the catalyst without wasting fuel or producing excess emissions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.